1. Field of the Invention
The invention relates to processing semiconductor crystal rods and somewhat more particularly to a method and apparatus for floating zone melt refining of a semiconductor crystal whereby dislocations and the like within the processed crystal are avoided.
2. Prior Art
Semiconductor crystals, particularly those composed of silicon, are generally produced by a floating zone melt refining process whereby a monocrystalline seed having a relatively small diameter is melt-connected, as with the aid of an induction heating coil, to an end of a relatively large diameter polycrystalline semiconductor rod and a melt zone is generated at the juncture of the seed crystal and the polycrystalline rod and passed one or more times along the length of the polycrystalline rod so as to convert the same to a monocrystalline state. The melt zone is moved by providing relative movement between the polycrystalline rod and a heat source, such as an induction heating coil, which may be the same one used to melt-connect the seed crystal with the rod or be different therefrom. In this manner, a polycrystalline rod is purified and converted into a monocrystalline member.
In the production of semiconductor components from so-produced semiconductor rods, it is desirable that the semiconductor rods be as free as possible from dislocations and other crystal lattice irregularities which interfere with electrical properties of the semiconductor components produced therefrom. Further, the presence of dislocations, etc. within the semiconductor material decreases the life of minority carriers within such semiconductor components.
German Auslegeschrift No. 1,079,583 (which generally corresponds to British Letters Pat. No. 889,160) suggests that dislocations in rod-shaped semiconductor crystals may be decreased at the melt-connected juncture of the seed crystal and such semiconductor rod by decreasing the cross-section of the semiconductor rod at the direct proximity of such melt-connecting juncture prior to the last pass of the melt zone through the semiconductor member. Dislocations which may be present in the seed crystal are thus given a chance to heal in the thus-produced thin connecting piece or bottleneck-shaped bridge between the seed crystal and the semiconductor member.
German Letters Pat. No. 1,128,413 (which generally corresponds to U.S. Pat. No. 3,175,891) discloses that substantially dislocation-free rod-shaped silicon monocrystals may be produced, for example, by controlling the rate or speed of travel of one or more passages of a floating zone melt through the rod. This reference suggests that all passages of the melt zone start in the seed crystal and that the travel speed of the melt zone in a seed crystal be controlled so as to be in the range of about 7 to 15 mm/min. During the last pass of the melt zone, the silicon rod cross-section at the junction of the seed crystal and a silicon rod is constricted by a temporary relative movement of the rod and at a speed greater than 25 mm/min., while the speed of the melt zone is steadily decreased from this constriction point until the full cross-section of the rod is again attained. Thereafter, the melt zone is moved through the rod at speeds less than about 7 mm/min.
It has been noted that when semiconductor rods of a fairly large diameter are being produced by the floating zone melt process, the rod-shaped monocrystals which grow at the seed crystal during the last passage of the melt zone tend to vibrate or oscillate, particularly at the thin-connecting or bottleneck-shaped bridging piece between the monocrystal and the seed crystal. This drawback is particularly acute when thick monocrystal rods are being produced. These vibrations appear to cause a development of dislocations and other irregularities in the monocrystal as the molten material becomes rigid during the last passage of the floating melt zone through the semiconductor rod. In addition, such oscillations often cause a dripping of molten material from the melt zone or even a breakage of the bottleneck-shaped bridging piece between the seed crystal and the semiconductor rod, which, of course, causes an interruption in the zone melt process.
I have disclosed in German Offenlegungsschrift No. 1,519,901 a means of supporting the ends of a crystal rod at the juncture thereof with a seed crystal which comprise a finger-like support means that is positioned on the upper edge of a casing and which is axially movable and encloses the mounting for the seed crystal. However, this arrangement does not completely obviate vibrations or oscillations during the growth of very thick (i.e., having a diameter larger than about 30 mm) dislocation-free semiconductor monocrystalline rods since the finger-like supports do not uniformly touch the overall round cone portion of such rod. Due to this instability, increased oscillations may be produced which oppose the supporting effect desired or may even eliminate any beneficial supporting action.
I have also disclosed in my above referenced earlier disclosures, a movable hollow funnel-shaped casing coupled to a lower rod mounting member and which is movable from a lower position to an upper position for encompassing a cone area of the rod being processed. The interior of the funnel-shaped casing is provided with an oscillation or vibration dampening means, such as particulate silicon, quartz, sand, metal, metal spheroids, a liquefied metal which solidifies after contact with the cone area of the rod or metal inserts which form a eutectic mixture with the molten material of the rod on contact and then solidified to provide support for the rod being processed. The use of funnel-shaped casing in this manner prevents vibrations and the like which typically occur when a melt zone on a semi-conductor rod being refined moves too far away (depending on the diameter being processed, about 50 cm) from the juncture of the seed crystal and the stock rod or, respectively, from the bottleneck-shaped bridging area between the seed crystal and the processed rod. It is hypothesized that the rod weight on the bottleneck area becomes too great and the funnel-shaped casing filled with the vibration dampening means relieves the rod weight and prevents dislocations and the like from occuring in the rod being processed.
In the system described in my earlier referenced disclosures, the supporting jacket is axially movable relative to the rod support by means acting thereon outside the zone melt chamber. In these systems, as the melt zone approaches a given distance from the juncture of the seed crystal and the stock rod (which distance is critical for the occurrence of vibrations and/or oscillations in the crystal lattice), the empty funnel-shaped jacket is moved upward so as to encompass the cone-like region of the processed rod situated above the seed crystal. Then, the stabilizing means is brought into contact with the cone region of the rod, as by filling the jacket with particulate silicon or the like. However, the jacket may also be constructed so as to function as a lifting means whereby the granular or spherical oscillation dampening material is in the jacket prior to the upward movement thereof.